Add the ability to configure the cache directory

At present the cache directory is hard coded:

```
def self.setup_default_cache_dir
  windows_dir = windows_data_dir
  @facts_cache_dir = if windows_dir
			 File.join(windows_dir, 'PuppetLabs', 'facter', 'cache', 'cached_facts')
		       else
			 '/opt/puppetlabs/facter/cache/cached_facts'
		       end
end
```

It would nice to be able to set the cache directory in `facter.conf`